1. Field of the Invention
This invention relates generally to a control system for an electric golf car and, more particularly, to a control system for an electric golf car that includes a regenerative braking system employing a half-bridge rectifier that provides regenerative braking to lower than base speed.
2. Discussion of the Related Art
All electric motors work on the principle that two magnetic fields in proximity will have a tendency to align themselves. One way to produce a magnetic field is to take a coil of wire and pass a current through it. If two coils with current passing through them are in proximity to each other, their respective magnetic fields will have a tendency to align themselves. If the two coils are between 0 and 180 degrees out of alignment, this tendency will create a torque between the two coils. If one of these coils is mechanically fixed to a shaft and the other is fixed to an outer housing, an electric motor is provided. The torque produced between these coils varies with the current in the coils.
Unfortunately, this motor will only turn half of a revolution before the fields line up. It is thus necessary to make sure that there is always an angle between the two coils so as to continue to produce torque as the motor shaft rotates through more than 180 degrees. A device that provides this function is called a commutator. The commutator disconnects the current from the active moving coil, referred to as the armature coil, and reconnects it to a second armature coil, before the angle between the armature coil and the field coil connected to the housing reaches zero. The ends of each of the armature coils have contact surfaces known as commutator bars. Contacts made of carbon, called brushes, are fixed to the motor housing. As the motor shaft rotates, the brushes lose contact with one set of bars and make contact with the next set of bars. This process keeps the angle between the active armature coil and the field coil relatively constant. This constant angle between the magnetic fields maintains a constant torque throughout the motor""s rotation.
If a coil is moved in a magnetic field, a voltage and current are induced in the coil. If a current passes through the field coil and the armature coil is turned, a voltage and current are induced in the armature coil, effectively turning the motor into a generator. This has two important effects. When the motor is used to power an electric vehicle, such as an electric golf car, referred to as motoring, the rotation of the motor induces a voltage across the armature coil called back EMF (electro-motive force). This voltage goes up with the speed of the motor, and also with the field current. When the back EMF equals the voltage across the terminals of the motor, the top speed has been reached. The other effect is that if an electrical load is placed on the armature coil, and the armature coil is turned, the motor will act as a brake and generate power. This effect is known as regenerative braking. This is an electric motor where the torque produced varies with the current in the armature and field coils, and the speed varies with the applied armature voltage.
Examples of this regenerative type braking for an electric golf car can be found in U.S. Pat. No. 5,565,760 issued to Ball et al.; U.S. Pat. No. 5,814,958 issued to Journey; U.S. Pat. No. 5,332,954 issued to Lankin; and U.S. Pat. No. 4,626,750 issued to Post.
The speed of an electric vehicle will vary by varying the voltage applied to the motor. With a lower voltage, the back EMF of the motor reaches the applied voltage at a lower speed. There are two different ways to vary this voltage. The first is to insert resistors in series with the motor to lower the effective voltage to the motor. This is the way the industry used to control motor speed. Unfortunately, this method is extremely inefficient at lower speeds.
This inefficiency can be explained by Ohm""s law and Kirchoff""s current and voltage laws. Ohm""s law states that:
V(Voltage)=I(Current)xc3x97R(Resistance)
from which:
P(Power)=I(Current)xc3x97V(Voltage).
Kirchoff""s law simply states that in a circuit, all the voltages must add up to zero, and all the currents must be the same in a given loop.
By Kirchoff""s current law, the current through the battery, the resistor, the armature coil, and the field coil in an electric vehicle motor circuit must all be the same. Also, by Kirchoff""s voltage law, the voltages across the resistor, the armature coil, and the field coil must all add up to the battery voltage (36 V in one example), so the sum of all of the voltages in the circuit equals zero.
Assume that certain driving conditions (grade, surface, tire pressure, load on the vehicle and desired speed) dictate that a current of 100 A at 18 V be across the motor (armature and field coils). The torque varies with current, and the speed varies with voltage. The circuit can be analyzed to determine how much power is lost in the resistor. By Kirchoff""s law, the voltage across the resistor is given as:
VBATT=VARM+VFIELD+VRES
36=18+VRES
VRES=18 Volts
The current is 100A, therefore by Ohm""s law the power lost in the resistor is given as:
PRES=IRESxc3x97VRES
PRES=100xc3x9718=1800 watts.
Also by Ohm""s law the power being used by the motor is:
P=(VARM+VFIELD)xc3x97I
P=18xc3x97100=1800 watts.
This means that half of the power coming out of the batteries is being lost to heat in the resistor. Under these conditions, the speed controller system uses half of the energy of the resistor system for the same performance.
In a resistor system, the resistance decreases as pedal position increases. In a speed controller system, the duty cycle increases as pedal position increases. Both ways effectively control the voltage to the motor, and therefore the speed of the vehicle. The difference in efficiency is less noticeable the closer to full throttle.
While conventional electric vehicles operate on the principles outlined above, there are different ways of controlling them. The standard electric golf car uses a series wound motor. A series wound motor has the field coils wound with a few turns of very heavy wire. In order to get maximum torque, the armature and field coils are connected in series. Other electric vehicles use shunt wound motors, where the field coil has many turns of smaller wires. In order to get maximum torque, the armature and field coils are connected in a parallel or xe2x80x9cshuntxe2x80x9d configuration. The strength of the magnetic field produced by a coil varies with the current passing through the coil and the number of turns in the coil. Therefore, the same field strength can be provided by passing less current through a shunt field winding. For example, the same field strength at 300 A in the series wound motor can be achieved with 15-20 A in the shunt wound motor. There are a couple of notable differences in the controller as well. Since less current is required to get the same field in a shunt wound motor, it gives the opportunity to control the field coil with a separate set of smaller power components. This is called separately excited control of the motor.
As discussed above, back EMF varies with the field strength, which varies with the field current. In a series wound motor, the armature current and the field current are the same, thus the relationship between the field strength and the armature current is a straight line. In a separately excited system, any field current can be chosen for a given armature current. As field current decreases, the field strength decreases. Thus, the back EMF is lowered, which increases the motor speed for a given armature current. This is called field weakening.
If the vehicle begins to roll backward down a hill while the field current is still active in the forward direction, it will generate current backward directly into the freewheel diode. Since the diode looks like a short circuit in that direction, this will cause the motor to act like a brake to a very slow speed. This type of braking is called plug braking.
If the vehicle has been stopped with the accelerator released for more than a predetermined period of time, the controller will de-energize the field coil and continue to monitor the speed sensor. If the vehicle begins to move without the accelerator being depressed, the controller will re-energize the field in the direction opposite of the vehicle motion to initiate plug braking.
In accordance with the teachings of the present invention, a control system for an electric vehicle, such as a golf car, is disclosed that includes a plurality of power metal-oxide semiconductor field effect transistors (MOSFETs). Four of the MOSFETs make up a full H-bridge, and two of the MOSFETs make up a half H-bridge. One of the MOSFETs in the half-bridge is connected in parallel across the armature coil and the other MOSFET in the half-bridge is connected in series with the armature coil. The parallel MOSFET across the armature coil includes a freewheel diode.
When the motor is operating below its base speed, i.e., when the back EMF is below the power supply voltage, a current flow can be induced by momentarily turning on the parallel MOSFET shorting the armature coil. Because of the inductance in the motor armature, a current flow in the counter clockwise direction is initiated at which point the series MOSFET is turned off. Consequently, a large voltage spike across the motor occurs which results in current flow back into the battery pack. By quickly turning the parallel MOSFET on and off using a technique referred to as pulse width modulation (PWM), the current flow can be maintained, and regenerative braking can occur even at slow speeds.
Additionally, control of the negative quadrant of the field map is possible due to the fact that the armature current is monitored. By varying the field current (If) to match a reference current (Iref), the field map curves can be maintained. Further, because motor speed varies with armature voltage (VARM), the armature PWM signal can be used to control vehicle speed as opposed to the method of field current variation speed control in previous controllers. Also, through a closed control loop, the speed reference value (determined by pedal position) is compared to the actual speed value returned by the speed sensor. Adjustments are made to the armature PWM signal to vary vehicle speed in accordance with a throttle position input signal. And, in order to prevent irregular regenerative braking characteristics during low speed maneuvering, the speed is monitored and regenerative armature current limit (Imax) is reduced at low speeds.